1. Technical Field
The present invention relates to an electrode structure with low resistance and high thermal stability which has completely suppressed the generation of a hillock and a whisker as stress migration, as an electrode wiring material for liquid crystal displays, and also relates to a method of forming the electrode structure.
2. Prior Art
Pure metal, such as Cu, Al, Mo, Ta, and W, and alloy material, such as Al--Cu, Al--Cu--Si, Al--Pd, have hitherto been employed as low-resistance electrode wiring materials that are employed in semiconductor devices. On the other hand, particularly an electrode material for a liquid crystal display, which has been given attention as a flat panel display, has recently been required to have better characteristics than before, such as large area wiring for large screens, high integrated wiring for high fineness, and array formation which is film formation onto a glass substrate. In FIG. 1 there is shown a schematic diagram of one pixel section of the array of a liquid crystal display which has a thin-film transistor (TFT) as an active device. A display electrode 2, a gate line 3, a gate electrode 3A, a data line 4, a drain electrode 4A, a source electrode 5, and an active TFT device 6 are disposed on a single pixel opening portion 1. If the TFT is turned on by a signal on the gate line 3, then the electric potential on the data line 4 will become equal to the pixel electrode 2 connected to the data line 4 through the source electrode 5. As a consequence, the liquid crystal, enclosed in the upper portion of the pixel electrode 2 in the paper surface direction, is oriented and the pixel is caused to be in a display state. Here, the electrode wiring materials for the array of a liquid crystal display to which the present invention is applied indicate the gate line 3, the gate electrode 3A, the data line 4, the drain electrode 4A, and the source electrode 5.
The first required characteristic of the electrode wiring material for liquid crystal displays is that the electric resistance is small. If the electric resistance is large, various problems such as signal delay and heat generation will arise when the screen of a liquid crystal display is enlarged. Pure aluminum with low electric resistance has often been employed as a wiring material for liquid crystal displays. Pure aluminum is excellent in etching characteristic and also is a suitable material from the standpoint of adhesion with respect to a substrate. However, pure aluminum has the disadvantage that the melting point is low and it will easily give rise to defects, called hillocks and whiskers, by a thermal process in chemical vapor disposition (CVD) after formation of a wiring film. This thermal process is usually carried out at a temperature of 300 to 400.degree. C. After this process, if the wiring material is observed by an electron microscope, there are cases where microscopic protrusions or bar-shaped crystal growth will be observed on the surface.
An example of defects such as this is shown in FIG. 2. Shown in FIG. 2 is a wiring layer 20 formed on a glass substrate 17. In general, a wiring layer consists of pure aluminum (Al) or the alloy and is constituted by some crystal grains 21 through 26. Here, a portion 30 extending lengthwise in the form of a whisker from the crystal grain 22 is called a whisker, and a portion 40 bulged from the crystal grain 24 is called a hillock. If the whisker 30 and the hillock 40 (hereinafter referred to as a hillock and the like) are generated, the smoothness of a wiring material layer will be lost, and a nitride film and an oxide film, which will be formed on the wiring material layer in a postprocess, will be formed on and along the underlying unevenness. Therefore, the generation of a hillock and the like will become an extremely large problem in the process of fabricating liquid crystal displays. The mechanism of the generation of a hillock and the like has not yet been settled, but it has been considered that if compression stress acts on a thin film by a difference in linear expansion coefficient between the thin film and the substrate due to heating, Al atoms will be moved toward the surface of a wiring layer along a grain boundary with this compression stress as a driving force, whereby a hillock and the like will be generated.
If high-melting point metal, such as Cr, Ti, Ta, and MoTa, is used in wiring material, the generation of a hillock can be prevented because the generation of atom diffusion along a grain boundary is difficult. However, these high-melting point metals are wiring materials which do not meet a tendency in increased size of the liquid crystal display, because the specific resistances generally are high like more than 50 .mu..OMEGA. cm (about 3 .mu..OMEGA. cm for aluminum).